In many applications there is a need for coupling high frequency (e.g., radio frequency (RF) or microwave) energy between two circuits while providing some measure of DC or low frequency electrical isolation between the circuits. Transformers are commonly employed to provide this function. One class of devices that may be employed for such a transformer include acoustic processing devices, such as a bulk acoustic wave (BAW) devices and thin film acoustic wave devices.
FIGS. 1A-B illustrate an exemplary acoustic power transformer 100. Acoustic power transformer 100 includes an input port 102, an output port 104, a transmitting acoustic transducer 110, a receiving acoustic transducer 120, and an acoustic wave propagation medium 130 disposed in an acoustic wave propagation path between transmitting acoustic transducer 110 and receiving acoustic transducer 120.
Transmitting acoustic transducer 110 and receiving acoustic transducer 120 may be piezoelectric devices adapted to convert a high frequency signal (e.g., an RF or microwave signal) to an acoustic wave, and vice versa. In one embodiment, transmitting acoustic transducer 110 and receiving acoustic transducer 120 each comprise a small “patch” of Lead Zirconate Titanate (PZT) material, for example 1 mm×1 mm square patch.
Acoustic wave propagation medium 130 may be a material such as alumina.
Input port 102 includes a first and second terminal connected, respectively, to first and second electrodes of transmitting acoustic transducer 110. Output port 104 includes a first and second terminal connected, respectively, to first and second electrodes of receiving acoustic transducer 120.
In operation, an input signal (e.g., an RF or microwave signal) is applied to input port 102 and thereby applied across the electrodes of transmitting acoustic transducer 110. In response to the input signal, transmitting acoustic transducer 110 generates an acoustic wave which is launched into the acoustic wave propagation medium 130. The receiving acoustic transducer 120 receives the acoustic wave and in response thereto, generates an output signal (e.g., an RF or microwave signal) which is applied to output port 104.
In this manner, acoustic power transformer 100 is able to transfer high frequency energy from input post 102 to output port 104 while maintaining DC isolation between the two ports.
However, acoustic power transformer 100 has some drawbacks.
In particular, when the acoustic wave is launched from transmitting acoustic transducer 110, in the “near field” near the transmitting acoustic transducer 110 it appears to have a planar wavefront for all intents and purposes. However, when the acoustic wave has propagated much further away from transmitting acoustic transducer 110, in the “far-field,” the acoustic wave assumes a spherical wavefront. So, as a simplification, we can consider that as the acoustic wave propagates away from the transmitting acoustic transducer 110, at first its wavefront is generally planar until it reaches a near-to-far field transition point beyond which the wavefront becomes spherical. For a square acoustic transducer 110 having an side length W, then the near-to-far field transition point is at a distance, P, from transmitting acoustic transducer 110:
                    P        =                                            (                              W                2                            )                        2                    *                      1            λ                                              (        1        )            where λ is the wavelength of the signal.
Accordingly, after passing the near-to-far field transition point, P, the acoustic wave has a spherical wavefront. However, receiving acoustic transducer 120 has a generally planar surface. As a result, as can be seen in FIG. 1, different portions of the wavefront of the acoustic wave reach receiving acoustic transducer 120 at different times. In other words, parts of the acoustic wave reach receiving acoustic transducer 120 out of phase with each other, resulting in self-interference and partial cancellation of the received acoustic wave at receiving acoustic transducer 120. For example, as seen in FIG. 1, when a first portion of the acoustic wavefront reaches receiving acoustic transducer 120, a second portion of the acoustic wavefront still has to travel a distance “d” before it will reach receiving acoustic transducer 120. Now if d is λ/2, then it is clear that the second portion of the acoustic wavefront reaches receiving acoustic transducer 120 exactly 180 degrees out of phase with the first portion of the wavefront, resulting in cancellation between the two portions.
Furthermore, due to the spherical spreading of the acoustic wave as it propagates, some of the acoustic beam will miss the receiving acoustic transducer altogether.
As a result of these two effects, the insertion loss of acoustic power transformer 100 is increased, resulting in a signal reduction at receiving acoustic transducer 120. Also, it can be seen form equation (1) above that this insertion loss decreases at higher frequencies.
What is needed, therefore, is an acoustic power transformer able to operate with lower insertion loss.